<!DOCTYPE html>
<html>
<head>
	<title></title>
	<script type="text/javascript" src="http://libs.baidu.com/jquery/2.0.0/jquery.min.js"></script>
	<style type="text/css">
		html,body{
			text-align: center;
		}
		canvas{
			border:1px solid red;
		}
	</style>
</head>
<body>

	<canvas id="c" width="800" height="600"></canvas>
	<script type="text/javascript">
		var ctx = c.getContext('2d');
		ctx.beginPath();
		ctx.arc(200,200,50,0,2*Math.PI,false);
		ctx.stroke();
	c.onmousedown = function () {
		var x1 = event.offsetX;
		var y1 = event.offsetY;
		ctx.beginPath();
		ctx.arc(x1,y1,50,0,2*Math.PI,false);

	}
	c.onmouseup = function () {
		ctx.stroke();
	}
	</script>
</body>
</html>